Processing facilities and other facilities routinely include tanks for storing liquid materials and other materials. For example, storage tanks are routinely used in tank farm facilities and other storage facilities to store oil or other materials. As another example, oil tankers and other transport vessels routinely include numerous tanks storing oil or other materials.
Often times, it is necessary or desirable to measure the amount of material stored in a tank. This may be useful, for example, during loading of material into the tank or unloading of material from the tank. As particular examples, “custody transfers” and “weights and measures of oil” often require highly accurate measurements from level gauging instruments installed on the roof of a tank. In bulk storage tanks, an error of one millimeter in a level reading can correspond to several cubic meters of volumetric error. This can result in losses of thousands of dollars for one or more parties.
One approach to monitoring the amount of material in a tank involves the use of radar measurements. In this approach, radar signals are transmitted towards and reflected off the surface of the material in the tank. For bulk storage tank level measurements, this approach often involves one of two measurement techniques: free-space techniques or stillpipe techniques (also referred to as stillingwell or standpipe techniques).
Stillpipe techniques typically involve the transmission of radar signals through a pipe within a tank. The pipe is often referred to as a stillpipe, stillingwell, or standpipe. For convenience, the term “stillpipe” refers collectively to stillpipes, stillingwells, or standpipes. A stillpipe typically includes a number of openings, such as holes or slots, on the stillpipe to allow material to flow into and out of the stillpipe. Because of this, the level of material in the stillpipe is generally equal to the overall level of material in the tank. As a result, the radar signals can be used to estimate the level of material in the stillpipe and therefore in the tank. One problem often encountered using radar transmissions in stillpipes is frequency dispersion. That is, the frequency of radar signals can change or disperse as the radar signals travel through a stillpipe.
One particular type of stillpipe technique involves determining the “group velocity” of radar signals transmitted through a stillpipe and performing multiple interpolations to identify the level of material in the stillpipe. However, this technique does not allow for mismatch between the inner diameter of the stillpipe and the diameter of an antenna used to transmit and receive the radar signals. For example, in applications such as custody transfer and weights and measures, the tolerance of mismatch must often be within one millimeter.
In fact, radar measurements taken using stillpipes are often degraded by the effects of mismatch between stillpipe inner diameter and antenna diameter. It is rather hard to make tens of meters of a pipe with such strict tolerance at a reasonable cost. Among other things, this mismatch typically increases frequency dispersion. This mismatch can also generate unwanted other modes or higher modes of electromagnetic fields, which can result in severe interference in main mode-based measurements and produce inaccuracies in the radar measurements.
In addition, in real-world operations, large stillpipe openings may be needed for viscous material to freely move from outside the stillpipe to inside the stillpipe and vice versa. However, the large openings on the sides of the stillpipes can degrade the accuracy of radar measurements even more. This is because the openings may generate drastic disturbances in electromagnetic fields propagating inside the stillpipes.
One approach to solving this problem involves imposing stringent requirements on the inner diameters of stillpipes. However, this can lead to high manufacturing and installation costs to ensure the accuracy of level radar gauges under the required stillpipe conditions.